There is a great deal of information in the literature, mostly in patents, concerning catalytic hydrogenolysis of sugars or sugar alcohols into mixtures of polyols, for example, glycerol, ethylene and propylene glycols. Hydrogenolysis involves the cracking of a carbon to carbon linkage in a molecule with the simultaneous addition of hydrogen to each of the fragments produced by the cracking. The hydrogenolysis of sugars and higher polyols has multiple steps. The polyol is first dehydrogenated by a catalyst to an aldehyde or a ketone. The product of dehydrogenation undergoes either a C—C or a C—O cleavage. The overall reaction sequence leading to C—C cleavage or C—O cleavage, which occurs by dehydration, is affected by a metal catalyst and a base and goes through a number of intermediates. The metal catalyst is both a hydrogenating and a dehydrogenating catalyst.
Catalysts based on Ir, Ni, Rh and especially Ru are active but poorly selective because they excessively hydrogenolyse C—C and C—O bonds. Modification of these metals by additives often leads to significant changes in selectivities. As an example, Ru catalyst after modification with sulfur compounds (sodium sulfide) and in the presence of a base (Ca(OH)2) at 250° C. under 140 bar of hydrogen, is able to convert 98% of glucitol into 91% of a mixture of ethylene glycol (26%) and 1,2-propanediol (65%) (U.S. Pat. No. 4,430,253).
Tin has a similar effect as sulfur compounds on the modification of ruthenium catalysts, preferably in the atomic ratio Ru/Sn from 2 to 1 (G. Gubitosa and B. Casale. U.S. Pat. No. 5,354,914). Hydrogenolysis of higher polyhydric alcohols in the presence of Ru/Sn catalyst increases the selectivity towards the production of lower polyhydric alcohols and keeps the formation of undesired products such as gaseous hydrocarbons to a minimum. A certain advantage of tin modified Ru catalyst is that the quantity of the catalyst required to reach conversion of polyhydric alcohols, comparable to sulfur—modified Ru catalyst, is several times lower.
U.S. Pat. No. 5,210,335 describes the preparation of lower polyhydric alcohols by catalytic hydrogenolysis of sucrose in an aqueous solution using a catalyst whose active material in the unreduced form essentially consists of 66.8 wt % of CoO, 19.2 wt % of CuO, 7.1 wt % of Mn3O4, 3.4% H3PO4 and 3.5% of MoO3. The hydrogenolysis reaction proceeds at reaction temperatures of 250° C. and pressures from 280 to 300 bar for 4.5 hours of a total reaction time. The reaction mixture contains 60 wt % of propylene glycol, 20 wt % of ethylene glycol and the rest other mono-, di-, tri- and tetrahydric alcohols. The conversion of sucrose is total.
U.S. Pat. Nos. 5,214,219 and 5,616,817 use mixed-oxide catalysts Co—Zn and Co—Cu—Mn—Mo for hydrogenolysis of glycerol to ethylene and propylene glycols.
The increased selectivity of hydrogenolysis of sugars or polyhydric alcohols to desired products such as ethylene and propylene glycols is also achieved by using the synergistic effect of various multimetallic catalysts, for example Ni—Re (U.S. Pat. Nos. 6,841,085 and 7,038,094), Ni—W—Cu, and Ni—Mo—Cu.
Metallic catalysts are obviously supported on carriers. The type of carrier (e. g. acid-base properties), its texture (surface area, porosity) and the dispersion of metals are also very important factors influencing the activity of the catalyst and the selectivity of the hydrogenolysis process. For example, the preferred support for the ruthenium catalyst is microporous carbon (U.S. Pat. No. 6,291,725).
A homogeneous process has also been described (WO 2005/051874). The reaction proceeds in the homogeneous liquid phase in the presence of a homogeneous ruthenium or osmium catalyst coordinated with tridentate phosphines to give a sugar conversion in an excess of 90% with greater than 70% selectivity to ethylene and propylene glycols. Good results may be obtained at a pressure below 70 bar. As indicated, the reaction process of WO 2005/051874 must necessarily proceed in homogeneous liquid phase, wherein the catalyst is fully dissolved and not merely suspended. Only after such reaction process, when the catalyst is removed from the reactor, it can be possibly immobilized on a support to assist its recovery.
The hydrogenolysis process is conveniently carried out in an aqueous reaction medium. However, a variety of other solvents may be employed. Alternative solvents include, e.g., ethylene glycol, C1-C4 monohydric alcohols, especially methanol. Use of protic solvents results in a higher conversion to ethylene glycol than does use of e.g. cyclohexane (U.S. Pat. No. 4,404,411).
It is important to maintain the pH range during the hydrogenolysis process. Maintaining the pH within the preferable range 9.0 to 11. (U.S. Pat. No. 4,476,331) is important to achieve product selectivity. Useful basic materials include alkali metal hydroxides and basic salts.
There exists therefore a need for a heterogenous catalyst with high activity and selectivity that can be, without reactivation, recycled back through the hydrogenolysis process without losing activity and selectivity. Such a recyclable catalyst would enable high yields of lower glycols at milder reaction conditions, and with a lower concentration of catalyst. Such a catalyst, is heretofore unreported in the literature.